Marx: ego is not a dirty word

What did marx say about human nature?

This issue came up in a recent discussion here but it also comes up all the time whenever you mention communism or socialism. Someone I spoke to recently said:

Even if the whole world was socialist or communist then it would eventually revert back to capitalism because that is human nature

And in a comment on this blog Mark Miller said:

The problem I see with Marxism is it assumes that human nature can be molded and changed. Where Marxism has been tried it runs into what I'd call "the wall of human nature". It doesn't achieve its stated goals, because imperfect people are involved in the enterprise. Inevitably the same qualities that Marxism tries to overthrow: greed, the hunger for power over others, end up controlling the enterprise. I think Lord Acton's maxim holds true no matter what structure is tried: "Power corrupts. Absolute power corrupts absolutely." Such efforts to create a society where everything is parceled out equally and no one has more power than anyone else is continually frustrated, because human nature keeps getting in the way.

I recognise these opinions as "popular wisdom" but wasn't actually sure what Marx did say about human nature, so I looked it up and here is part of what I found, part of a polemic against Max Stirner:

"Communism is simply incomprehensible to our saint because the communists do not put egoism against self-sacrifice or self-sacrifice against egoism nor do they express this contradiction theoretically either in its sentimental or in its high-flown ideological form; on the contrary, they demonstrate the material basis engendering it, with which it disappears of itself. The communists do not preach morality at all, such as Stirner preaches so extensively. They do not put to the people the moral demand: love one another, do not be egoists, etc.; on the contrary, they are well aware that egoism, just as much as self-sacrifice, is in definite circumstances a necessary form of the self-assertion of individuals. Hence, the communists by no means want,... to do away with the "private individual" for the sake of the "general," self-sacrificing man....

Communist theoreticians, the only ones who have time to devote to the study of history, are distinguished precisely because they alone have discovered that throughout history the "general interest" is created by individuals who are defined as "private persons." They know that this contradiction is only a seeming one because one side of it, the so-called "general," is constantly being produced by the other side, private interest, and by no means opposes the latter as an independent force with an independent history - so that this contradiction is in practice always being destroyed and reproduced. Hence it is not a question of the Hegelian "negative unity" of two sides of a contradiction, but of the materially determined destruction of the preceding materially determined mode of life of individuals, with the disappearance of which this contradiction, together with its unity, also disappear."
- Personal, versus General Interest, from Selections from the remaining parts of The German Ideology by Marx and Engels